Information display devices are frequently used in harsh, hazardous, or extreme environments including marine, avionics, and industrial applications. In these harsh environments, the information display device is subjected to shock, vibration, impacts, water, and other elements that adversely affect the performance of the display device and may cause complete malfunction of the display.
Information display devices may also be used in an environment of high ambient light, such as direct sunlight, which can make the display device difficult to view. The viewing performance of information display devices in high ambient light conditions can be improved by placing an anti-reflective (“AR”) medium in front of the display device in order to attenuate the back-reflected ambient light. This anti-reflective medium typically takes the form of a rigid optical cover panel, or a touch screen made of glass or optically clear polymer plastic. A rigid optical cover panel of this form provides good scratch-resistance and protection from moisture and debris.
Several bonding processes have been created in order to attach a rigid optical cover panel to an information display device. One such process using epoxy cement provides sufficient bond strength, but lacks the necessary flexibility to allow different coefficients of thermal expansion between the rigid optical cover panel and the information display device. This typically results in mechanical stress within the assembly under temperature changes, imparting stress forces to the display device causing visual anomalies and luminance irregularities in the operating display. When subjected to a significant sudden thermal shock or repeated moderate thermal shocks, the adhesion of hardened epoxy can fail entirely, creating a permanent de-lamination of the assembly. Additionally, an epoxy bonding medium lacks the ability to absorb or dampen mechanical shocks from the cover panel to the display device which can appear in the form of flashes of light or other anomalies on the display device.
Another bonding process, using urethane as the bonding medium, has the inherent and unacceptable characteristic of yellowing with age. Also, urethane bonding materials of optical quality are quite expensive.
Another bonding process, using silicone bonding media, is resistant to yellowing, and mixtures of optical silicones have been demonstrated to be sufficiently flexible and pliable to allow various coefficients of thermal expansion of the optical panel and the display device front surface without inducing stress to the display device, Additionally, a flexible silicone adhesive is able to dampen mechanical shock to the display device. However, using silicone as a bonding medium has the inherent problem that silicone does not adhere well to polymer plastic, particularly acrylic and polycarbonate forms of polymer plastic which are the two most common, optically-clear plastics. This is because the low surface energy of the polymer plastic provides only a very weak bond. When the silicone bond is subject to mechanical stress such as a shearing force due to differing rates of thermal expansion of glass and polymers, or twisting of the assembly, or shock and vibration then de-lamination is the normal result. Therefore, the use of a shatter-resistant polymer as a display cover plate has not been feasible when optically bonding to a display device. Glass, which has been traditionally used in place of polymer plastic as an optical cover panel is vulnerable to impacts and cracking.
Another bonding process includes the use of an adhesive gasket between an optical cover panel and the display device in place of an optical bond. This technique results in an air gap between the display device and the plastic cover panel. This air gap causes a severe optical refractive index mismatch which degrades the overall appearance of the display. Additionally, the air gap is prone to moisture condensation.
What is needed are high strength optical bonding methods for forming assemblies such as information display devices having polymer plastic front cover plates.